1. Field of the Invention
The present invention relates to an improved process for the preparation of an alternating copolymer of propylene and butadiene. Although the wide general utility of cis-1,4-polybutadiene which is a synthetic rubber having structural regularity is widely known, it is considered to have some limitations in the field of automobile tires because of its chipping and cutting phenomenon and because of its low skid resistancy. Also, though the properties of cis-1,4-polyisoprene which is another synthetic rubber having structural regularity, as its structure suggests, resembles closely to those of natural rubber, it may not be regarded as advantageous with respect to a stable supply of the raw material and is generally regarded as a poor candidate for a synthetic rubber with a broad spectrum use.
On the other hand, in pace with the wider use of radial tires for automobile and the wider use of heavy weight cars, the demand has increased for rubbers having structural regularity such as natural rubber and cis-1,4-polyisoprene rubber which have properties required for such uses, and the development of these rubbers and other synthetic rubbers having structural regularity with similar structures is greatly needed.
Some of us have made an extensive and scrutinous study of developing a process for the preparation of new alternating copolymer rubbers from propylene and butadiene which are both commercially available in abundance, and they have disclosed a number of processes for the alternating copolymerization of an .alpha.-olefin and a conjugated diene with a catalyst system comprising a vanadium or a titanium compound as a transition metal compound. For example, some of us already disclosed that, in the case of the alternating copolymer of propylene and butadiene, when prepared with a vanadium-type catalyst, the micro-structure of the butadiene units in the alternating copolymer has a trans-1,4-configuration, and cristallizaton can be induced by stretching to give excellent strength properties to the copolymer; while that prepared with a titanium-type catalyst has an uneven micro-structure and shows a far excellent low temperature properties.
The properties of the alternating copolymers obtained by these processes have also been extensively investigated, and the evaluation of the rubbery properties of the propylene-butadiene alternating copolymer was reported by some of us in The International Rubber Conference (held in Tokyo, October, 1975) Text, p 13, and by Furukawa in Journal of Polymer Science, Symposium, Vol. 48, pp. 19-31 (1974), and their excellent properties have already been fully pointed out.
In line with the recent trend toward energy saving, research effort has been directed recently to an establishment of a polymerization process which allows a shift from solution polymerization to bulk polymerization (as liquid, solid or gas phase). Although a few of such attempts has been successful, many such project have not yet been realized because of difficulties which arise on converting a process from solution polymerization to bulk polymerization. Examples of the difficulties which hinder the realization of bulk polymerization include: lowering of catalyst activity, gel formation during polymerization, difficulties in the control of reaction or formation of by-products because of an unefficient removal of the heat of the reaction due to an excessive rise in viscosity.
The present invention relates to a process suitable for an alternating copolymerization reaction between propylene and butadiene by bulk polymerization.
2. Description of the Prior Art
Some of us have already disclosed many processes for alternating copolymerization of an .alpha.-olefin and butadiene by a catalyst system consisting mainly of a vanadium compound as the transition metal compound.
Representative catalyst systems for the processes include:
(1) A catalyst system comprising organoaluminum compound--vanadium(V) chloride (Japanese Patent Publication No. 43088/71, See U.S. Pat. No. 3,652,518);
(2) A catalyst system comprising organoaluminum compound--vanadium(VI) chloride (Japanese Patent Publication No. 43089/71, see U.S. Pat. No. 3,652,518)
(3) A catalyst system comprising organoaluminum chloride compound--vanadium compound (Japanese Patent Publication No. 16182/72);
(4) A catalyst system comprising organoaluminum compound--vanadium compound--chromyl chloride (Japanese Patent Publication No. 43090/71);
(5) A catalyst system comprising organoaluminum chloride compound--alkoxyvanadyl chloride (Japanese Patent Publication No. 16183/72);
(6) A catalyst system comprising organoaluminum compound--vanadyl alkoxide--chromyl chloride (Japanese Patent Publication No. 16184/72, see U.S. Pat. No. 3,652,518);
(7) A catalyst system comprising organoaluminum compound--vanadium complex compound--chloroalkane (Japanese Patent Publication No. 16185/72);
(8) A catalyst system comprising organoaluminum compound--vanadium complex compound--thionyl chloride (Japanese Patent Publication No. 8611/72);
(9) A catalyst system comprising organoaluminum compound--VCl.sub.4 --compound containing OH group, P or S (Japanese Patent Publication No. 8615/72);
(10) A catalyst system comprising organoaluminum compound--vanadium alkoxide--halogen containing compound (Japanese Patent Publication No. 8616/72, see U.S. Pat. No. 3,652,518);
(11) A catalyst system comprising organoaluminum compound--vanadium complex compound--halogen containing compound (Japanese Patent Publication No. 16656/72);
(12) A catalyst system comprising organoaluminum compound--compound having Ti-X bond--compound having V-OR bond (Japanese Patent Publication No. 9412/72, see U.S. Pat. No. 3,652,518);
(13) A catalyst system comprising organoaluminum compound--compound having V-X bond--compound having Al-OR bond (Japanese Patent Publication No. 9413/72, see U.S. Pat. No. 3,824,224);
(14) A catalyst system comprising organoaluminum compound--compound having V-X bond--compound having hydroxy group (Japanese Patent Publication No. 19694/72);
(15) A catalyst system comprising organoaluminum compound--compound having V-X bond--compound having metal-OR bond (Japanese Patent Publication No. 26871/72, see U.S. Pat. No. 3,824,224 and
(16) A catalyst system comprising organoaluminum compound--compound having V-X bond--compound having metal-OR bond--metal carboxylate (Japanese Patent Publication No. 28715/72, see U.S. Pat. No. 3,824,224).
However, these catalyst system are suitable for performing the polymerization reaction in the presence of an aliphatic saturated hydrocarbon such as hexane, heptane, octane, etc., an aromatic hydrocarbon such as benzene, toluene, xylene, etc., or a halogenated hydrocarbon solvent such as methylene chloride, tetrachloroethylene, chlorobenzene, etc.
When solvent-free catalyst components are mixed and especially when organoaluminum compound is mixed in the absence of solvent, an exothermic reaction takes place to give an ununiform catalyst system, and it results in lowering of the yield of the alternating copolymer and hinders the desired alternating copolymerization reaction by causing gel formation, formation of polybutadiene, propylene-butadiene block copolymer and in some cases, polypropylene by-products; and the properties of the copolymer thus prepared is often of low quality.
As described above, in the use of the vanadium catalyst for the preparation of an alternating copolymer of an .alpha.-olefin and a conjugated diene which some of us have already disclosed, the preparation of the said catalyst requires the use of a solvent in order to reduce the heat of reaction and also requires the use of a considerable amount of solvent during the polymerization to reduce the viscosity of the polymer solution formed during the polymerization. The use of a solvent suffers from disadvantages because it requires a purification process of the solvent to remove impurities which are usually always present in the solvent and the re-use of a solvent of polymerization necessitates its recovery and purification processes.